Battery power backup ???

Does anyone know a circuit wich will be able to keep a charge level acceptable, ie no damage and fit, to a battery that will be used as backup in case of main shutdown?

Basically a small integrated ups ?

The idea is to keep a battery fully charged while running on AC and be able to keep on running without any interruption what ever happens......

Got the idea ???

OK, found one circuit : here

Alternatively buy a small commercial emergency light or alarm system UPS that runs at 12v and hack into its power supply.